Ionization vacuum pressure gauges can be used in a wide variety of applications such as semiconductor manufacturing, thin film deposition, high energy physics, ion implantation, and space simulation. Ionization gauges can include both cold cathode ionization gauges (CCIGs) and hot cathode ionization gauges (HCIGs), and some example HCIG designs include Bayard-Alpert (BA), Schulz-Phelps, and triode gauges.
The sensor of a typical hot cathode ionization vacuum pressure gauge, shown in FIG. 1, includes a cathode 130 (the electron source, also called the filament), an anode 132 (also called the grid), and an ion collector electrode 134. For the BA gauge, the cathode 130 is located radially outside of an ionization space (anode volume) defined by the anode 132.
The ion collector electrode 134 is disposed within the anode volume. Electrons travel from the cathode 130 toward and through the anode 132, and are eventually collected by the anode 132. In their travel, the electrons impact molecules and atoms of gas, constituting the atmosphere whose pressure is to be measured, and create ions. The ions created inside the anode volume are attracted to the ion collector electrode 134 by the electric field inside the anode, thereby producing an ion collector current 104.
The pressure P of the gas within the atmosphere can be calculated from ion and electron currents by the formula P=(1/S)(ii/ie), where S is a scaling coefficient (gauge sensitivity) with the units of 1/torr and is characteristic of a particular gauge geometry, electrical parameters, and pressure range; and ii is the ion collector current and ie is the electron emission current.
The expected dynamic range of ion collector current ii may span several decades. Measuring a current with such a wide dynamic range can present several challenges, including, but not limited to, operational discontinuities, higher cost, increased complexity and undesirable calibration demands.
For example, a logarithmic amplifier may be used to compress the dynamic range of a current into a compressed logarithmic range, but calibration of logarithmic amplifiers may be time-consuming and may add complexity to the ion collector current measurement system. Further, since logarithmic conversion is an old technology, the number of log converter devices on the market is small and they tend to be expensive.
In another example, a programmable-gain amplifier can apply different gains to the input current, depending on the magnitude of the input current. But discontinuities in the amplifier output may occur when the amplifier gain switches from one value to another, creating undesirable gaps in the current measurement.